To give a feeling, how hard this puzzle is:I looked at two of the 17 steps solutions my program found. In both it needed 10 eliminations, before the first time another candidate could be kicked off with basic methods! (I dont know, how much of the eliminations are really necessary.) There are so much possibilities to combine eliminations (in average there are about 10 possible eliminations for each of the 10 first steps, but there are big differences), that it is impossible for me, to optimise the order in reasonable time (without big effort).

To compare: in the (first) 9.9 puzzle (8 steps) of Ocean's recent list already after the second elimination other candidates fell out.

BTW: in the 2 17-steppers only 2 of the eliminations are common (r2c9<>7,r6c3<>2)